1. Field of the Invention
The present invention relates to the area of methods for inerting tanks used for the storage or also for the preparation of products and is concerned particularly with tanks described as “aseptic”.
2. Related Art
Document WO03/070024 illustrates this technical area.
More generally, in numerous industries, it is necessary to prepare or to hold products in storage in tanks or containers under controlled atmospheres with a low residual oxygen content. This requirement may result in variable stresses depending on the nature of the products in storage, and associated in particular with reasons of safety and with the desire to preserve the quality of the product, because of the susceptibility of the products in storage to oxidation, etc.
This holding of such stocks under an atmosphere with a low concentration of oxygen is commonly referred to as “inerting”.
Inerting thus consists of replacing a volume of air present inside the tank to be inerted by a volume of a gas considered to be inert under the conditions of use in question. The gases that are used most frequently in such operations are nitrogen, argon or carbon dioxide, or mixtures thereof.
The techniques most commonly used to inert such tanks, and thus to replace air by inert gas, include:                Purging by a piston effect. This technique involves introducing the inert gas at a point opposite the outlet for the purged (evacuated) gas. This technique is used in particular for small cross sections, and very rarely for tanks, due to obvious constraints relating to the dispersion of the gas. The quantity of inert gas consumed is close to one volume of gas per volume of gas to be purged.        Inerting by dilution. This technique is widely used, and it is suitable for capacities of all sizes. It involves injecting the inert gas through an orifice and then proceeding with the successive dilution of the air that is present inside the tank. The said dilution is maintained until the desired concentration of residual oxygen is reached.        
The quantity of inert gas consumed by this technique is close to 3 volumes of gas per volume of capacity to be inerted, in order to achieve a concentration of residual oxygen typically between 2 and 5%.                Inerting by compression and expansion. This technique is very rarely used to inert a tank. In fact, this technique requires either the pressure of the tank to be lowered in order to eliminate a proportion of the air that is present, or the pressure of the tank to be increased in order to facilitate the dilution. However, tanks are very rarely suitable for the required working pressures.        
The quantity of inert gas consumed by this technique depends on the number of cycles used. It is often in the order of between 1 and 2 volumes of inert gas per volume of the tank.
Let us now consider the particular case of tanks described as “aseptic”.
Certain industries, such as the agri-food or pharmaceutical industry (for obvious reasons that we will not explore in greater detail here), are obliged to undertake regular washing of their tanks in the context of the aseptisization of these tanks.
In the majority of cases, aseptic washing involves washing the tank with a hot alkaline solution at a temperature close to 80° C. This is followed immediately afterwards, and without waiting for the tank to cool down, by rinsing the tank with cold water. This cold water (10 to 25° C.) is injected through the same projection elements (of the washing balls type) that are situated at the top of the tank and were previously used to supply the hot washing solution.
It will be appreciated that this thermal cycling brings about a reduction in the pressure of the tank, which reduction in pressure sucks in external air.
More precisely, during the hot washing step, the atmosphere of the vessel is filled with a gaseous mixture comprising the gas that is initially present inside the vessel, this being either air or an inert gas if the tank had been inerted initially, and with water vapor, of which the percentage depends on the temperature (this can be appreciated more readily below from the curves).
Subsequent rinsing of the vessel with cold water causes the water vapor to condense, thereby bringing about a reduction in pressure if the tank is closed, or the aspiration of a gas, in this case air, if the vessel is open to the atmosphere.
In the case of tanks of which the atmosphere is controlled (inerted), this venting is often effected by opening a vent permitting the admission of air during the rinsing phase, the consequence of which is to cause the admission of a large volume of air, which it is necessary to eliminate at a later stage by one of the conventional inerting techniques already mentioned above.
The use of conventional inerting techniques require:                A significant down time for the vessel to bring the concentration of oxygen down to a sufficiently low value.        A significant consumption of gas.        Haphazard management of the inerting process without the excessive consumption of inert gas, except for controlling the atmosphere by analysis of the residual oxygen, which represents a costly solution, in particular in the case of a plurality of vessels requiring to be inerted.        